Transmit receive switching system for pulse communication systems



9-, 19 L. D. SMULLHN E! AL ,4 9 THANSMIT RECEIVE SWITCHING SYSTEM FOR PULSE COMMUNICATION SYSTEMS Filed April 18, 1944 WITHOUT HIGH Q CAV/ TY POWER r0 c/wsru rw/vsmrrm ANTENNA SWITCH HIGH '6! mw/rr cm/s m1. I MIXER I nsci/ v51? ii: WITH HIGH a W Tr POWER TO awsmz.

INVENTOR$ LOU/5 0 SMULL/N JEROMEB W/ESNER Patented Aug. 9, 1949 TR NSM E RECEI $W ..T= H. .G STEM For P J W9MM QT 9N S s-Hrs Louis D. Smullin, Arlington, and J GPOmB'BuWiBS- 'ner, Boston, Mass, ,assignors; by mesne assign.- ments, to the :United States-of America as genresent d by the .asec 'eta 'y of Wa trnlicatiog April L8, 1944, Serial No. 531,654

This invention relates -to a communication system and more particularly to a system of this character wherein radio transmitters and re ceivers operate in close proximity to each other. In many systems, such as radar systems, it is customary to pulse a transmitter at -fixed' intervals and utilize received echoes for determination of target data. Such systems may have separate or common antennae.

In a system of the above character, the amount of power radiated from the antenna during transmitter operation is extremelylarge in comparison to the amount of power necessary 101 receiver 015- era-tion. Irrespective of whether the transmitter and receiver have a common antenna or separate antennae, the problem of protecting the receiver from excessive energy input during transmitter opera-tion is present. In particular where the transmitter and-receiver-have an antenna in common and may in fact be connect-ed 'in the same communication channelto suc'han'antenna, itis necessary to provide suitable switching means for protecting the receiver during transmitter operation.

One manner in which .a receiver may be protected involves the use of an overload switch comprising a gaseous discharge gap which is adapted to be broken down and thus alter the transmission characteristics of the communication channelbetween receiver and antenna. Such .6 .Claims. (01. mean) a system isparticularly useful wherethe communication channel is in the form of a wave guide. A switch of this character is disclosed in the co-pending application of Harold K. Farr, Serial Number 488,098, filed May 22, 1943. "Thus by changing the effective resistanceacross a portion of guide, it is possible-to accomplish a switching action which may cut off the receiver during transmitter operation and cut in the receiver at other times.

A switching action as described above in general relies upon a potential difference due to the wave front generating or increasing the number of ions in the gapzso that a gaseous discharge is effected. Even with the extremely steep wave fronts employed, it has been found that the mechanism of ionization for breaking down the gap requires a periodof time which isappreciable in comparison to the variation of voltage with time. Thus it frequently occurs that bef olt fihe switch or gap'has time to act, a damag n fim i lnt of energy'has passed on to the receiver. This amount of energy may temporarily or permanently damage certain parts, particularly crystal mixers or detectors.

thinnes frece v 'erclrystal, magm -my limit itslus'eful iife.

"T e invention hereinafter described provides a me us for protecting the receiver .and .allied equipment and preventing a-dangerous wave-rront ironi be'ing 'pa'ssedto the receiver during the time whichis required iorthe switch to change its condition. Tillie invention in general comprises .the insertion into the 'line between-receiver and switch of'a 'reacta'nce element having ilowuresistance.

$uc'h a 'reaetance element may vtake-.tlfie form of cavity resenator'thaving a satisfactory Q. it is well' known in -this artithat .=the Q,,of.any idevicaor circuit may be definedlas the ratio of energ stored to energy dissipated. The Tefiect era 'high'Q-reactance isr-to utilize the initial rburst of energy for generating-oscillations .thereingmd such energy .is givenout with" little less but at 'a r-educed level and .over .a longer period of time. l h'e efficiency or Q of the ireaetance .will de- "me what inigh-t be termed a distorting .acti 1 or the voltage front on the receiver. In general, the Q should be high enough so that "protection afforded the receiver .will endure for just -.:the period of time required for switch 0 tion. However, care .must exercised .to --p rev'enttoo high a-reactancefrom being inserted. flihusin-theuseof radar systems, .theisharpaiulses :reprcse'nt video ;bands of ifr'equency which miis't-be preserved. :fillheus'e ofreactances having excessively large Qs' willtend .to-reduce lthelvideo range and Ithus impair the sharpness of pulses to be received.

"in the drawing, Fig. 1 is a time-power "curve showing the energy passed by the usual switchi i e ris ei sbteie d hiei 1 i t ir vention'.

*"Beirringto Fig.-'1,itwill be seen-that asharp ie gy pea t passes through the usualiswitchinig 'dijice thei -beg nning of a" transmitted fpulse. This curve levels ori"as s hewn at ii -w-hen the changed elect al conditionscause switch blier tionl This 1 a1 peak 5', however, generally iitude t cae etiama'gelmth fer-ring to the book diagram of 2; it will be seen-that a' actance'lernent, shown time ragga as h iQ cayityfi islconnected between the "switch and the' crystal of the receiver. It is well known that the time necessary to build up energy in a cavity is equal to the time required for the stored energy to die out 3 and is a function of the decrement of the cavity or an inverse function of the Q.

The cavity or other reactanoe element may have, in actual practice, a Q of the order of 200 or 300 and may go higher, if the video range is not too great.

With a reactance of this character, as disclosed, the energy stored therein will be given out at the time the switch operates so that in effect the energy peak will be broadened out and reduced. The distance between the cavity and switch on the one hand and between cavity and receiver on the other hand in terms of wave length is not important. If tuning with the cavity is to be utilized, then the disposition of a reactance cavity must be carefully determined.

What is claimed is:

1. In a system having a guide for electromagnetic waves into which are introduced at different times relatively high power and low power pulses of electromagnetic energy, receiving means for said low power pulses connected to said guide by electromagnetic wave conducting means including switch means interposed between said receiving means and guide, said switch means being responsive to any one high power pulse and effective after an interval following initiation of said one pulse which is short compared to the length of said one pulse, to reduce greatly the transmission of energy therethrough, and a frequency selective filter means connected between said switch means and receiving means to permit only a portion of the energy passing through said switch means during said interval to pass through said filter means.

2. In a system having a guide for electromagnetic waves into which are introduced at different times relatively high power and low power pulses of electromagnetic energy, receiving means for said low power pulses connected to said guide by electromagnetic wave conducting means including switch means interposed between said receiving means and guide, said switch means being responsive to any one high power pulse and effective after an interval following initiation of said one pulse which is short compared to the length of said one pulse, to reduce greatly the transmission of energy therethrough, .and a cavity resonator which acts as a frequency selective filter interposed between said switch means and .receiving means to permit only a portion of the energy passing through said switch means during said interval to pass through said resonator.

3. In a system having a guide for electromagnetic waves into which are introduced at difierent times relatively high power and low power pulses of electromagnetic energy, receiving means for said low power pulses connected to said guide by electromagnetic wave conducting means including a first cavity resonator interposed between said receiving means and guide and having therein a pair of electrodes forming a discharge gap across which a discharge is established by any one high power pulse after an interval following initiation of said one pulse which is short compared to the length of said onepulse, said discharge being efiective to reduce greatly the transmission of energy through said resonator, and a second cavity resonator which acts as a frequency selective filter interposed between said first resonator and receiving means to permit only a portion of the energy passing through said first resonator during said interval to pass through said second resonator.

4. Apparatus for use in a system having receiving means for low power pulses conductively connected to transmission means into which are fed at difierent times high power and low power pulses of electromagnetic energy, comprising switch means adapted to be interposed in the connection between said transmission means and receiving means, said switch means being of the type responsive to any one high power pulse to reduce substantially the transmission of energy therethrough after an interval following initiation of said one pulse which is short compared to the length of said one pulse, and a resonant circuit coupled to said switch means to be interposed between said switch means and receiving means in said connection, said resonant circuiti having a frequency band Width greater than the band width of the low power pulse but less than the band width of that portion of the high power pulse energy that is passed by said switch means.

5. Apparatus for use in a system having receiving means for low power pulses conductively connected to transmission means into which are fed at different times high power and low power pulses of electromagnetic energy, comprising circuit means interposed in the connection between said transmission means and receiving means responsive to any one high power pulse to reduce substantially the transmission of energy therethrough after an interval following initiation of said one pulse which is short compared to the length of said one pulse, and a resonant circuit coupled to said circuit means and interposed between said circuit means and receiving means in said connection, said resonant circuit having a frequency band width greater than the band width of the low power pulse but less than the band width of that portion of the high power pulse energy that is passed by said circuit means.

6. In a system having a guide for electromagnetic waves into which are introduced at different times relatively high power and low power pulses of electromagnetic ener y, receiving means for said low power pulses, electromagnetic wave circuit means connected between said receiving means and guide, said circuit means being responsive to any one high power pulse and effective after an interval following initiation of said one pulse which is short compared to the length of said one pulse, to reduce greatly the transmission of energy therethrough, and a frequency selective filter means connected between said circuit means and receiving means to permit only a portion of the energy passing through said circuit means during said interval to pass through said filter means.

LOUIS D. SMULLIN. JEROME B. WIESNER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,097,272 Drake Oct. 26, 1937 2,415,962 Okress Feb. 18, 1947 

